1. Technical Field of the Invention
This invention relates to a transmitting circuit for a mobile communication equipment having an automatic output power control circuit (APC), and particularly relates to a transmitting circuit having a directional coupler capable of detecting power outputted from a final stage amplifier and a reflected wave generated by the mismatching between the output impedance of the final stage amplifier and a load impedance connected to the output side of the final stage amplifier, and allowing the gate voltage of the final stage amplifier to be switched depending on the amplitude of this reflected wave.
2. Description of the Prior Art
As a radio frequency power amplifier used in the transmitting circuit of a conventional transmitter, Japanese Patent Application Laid-Open No. 57-118412, for example, discloses a radio frequency power amplifier wherein reflected power from the load of variable gain power amplification means of which gain varies with an external control voltage is detected by detection means. The reflected output is converted by DC conversion means. Further, the highest voltage is selected from among the converted voltages by selection means. The selected voltage is inputted into a comparator which determines the starting point of the gain control of the variable gain power amplification means. The converted voltage is outputted to control means by switch means, when the converted voltage exceeds a voltage set by the variable gain power amplification means, and this control means controls the gain of the variable gain power amplification means.
Further, FIG. 5 is a block diagram showing the constitution of a transmitting circuit for a conventional mobile communication equipment or the like.
In FIG. 5, a gate voltage 20 is applied to a gate of a final stage amplifier 21 amplifying a radio frequency signal and the output side of the final stage amplifier 21 is connected to an antenna 24 through a directional coupler 22 and an isolator 23.
Out of two detection outputs of the directional coupler 22, one detection output is grounded through a terminating resistor 25, while the other detection output inputs a detection voltage into the positive input terminal of a comparator 27 through a detection diode 26.
The detection diode 26 detects the output or reflected wave of the final stage amplifier 21 obtained from the directional coupler 22 and converts the detected output or reflected wave into a DC voltage, and the comparator 27 compares this DC voltage with a threshold voltage 28.
This threshold voltage 28 is applied to the negative input terminal of the comparator 17. The threshold voltage 28 is determined in advance by the characteristic of a load impedance connected to the output side of the final stage amplifier 21 and the reflected wave which is converted into a DC voltage by the detection diode 26.
The comparator 27 compares this threshold voltage 28 with the DC voltage of the detection diode 26, outputs an "H" level or an "L" level depending on whether or not the DC voltage is higher than the threshold voltage 28, supplies the output to an APC (circuit 29 and allows the APC circuit 29 to control the final stage amplifier 21.
In such a conventional transmitting circuit, the isolator 23 is inserted into the output side of the final stage amplifier 21, whereby the load impedance of the final stage amplifier 21 is intended to be stabilized and stable output distortion characteristic and consumption power are ensured.
Due to this, there is no avoiding outputting transmission power including the insertion loss of the isolator from the final amplifier so as to output specified transmission power from the antenna. The conventional transmitting circuit is not, therefore, suited for the reduction of consumption power.
Furthermore, since the isolator is a large component, the isolator is disadvantageously unsuited for miniaturization.
Meanwhile, the reduction of consumption power and the miniaturization of mobile communication equipment are increasingly underway in recent years. In these circumstances, it is demanded to secure the stable output distortion characteristics of a final stage amplifier and consumption power and to realize low power consumption and miniaturization.